1. Field of the Invention
The present invention relates to a resin laminate, and a process for its preparation. More particularly, it relates to a resin laminate of a polyolefin composition and a polyamide resin, a polyester resin or a saponified ethylene-vinyl acetate copolymer, having improved adhesion strength and a process for its production.
2. Description of the Prior Art
Conventional polyamide resins, polyester resins and saponified ethylene-vinyl acetate copolymers have excellent gas barrier properties, oil resistance, mechanical strength, etc. However, they have drawbacks that they are expensive and they have high water-permeability. These resins are not economically competitive with vinyl chloride resins which are inexpensive, although their gas barrier properties and oil resistance are superior to the vinyl chloride resins. As such they have been rarely used as packaging materials for food products. In recent years, a vinyl chloride monomer remaining in a vinyl chloride resin has been made an issue, and as a substitute for the vinyl chloride resin as a packaging material for food products, an attention has been drawn to a laminate of a polyolefin resin and a polyamide resin, a polyester resin or a saponified product of ethylene-vinyl acetate copolymer. This laminate has gas barrier properties, oil resistance and water-impermeability at the same time, and, as such, is useful for e.g. a container for food products such as mayonnaise or sauce, whereby it is possible to store such food products for a long period of time. On the other hand, for a use other than the use as a packaging material for food products, the polyolefin resin was not suitable for use as a container for gasoline because of the poor oil resistance. However, by making it into such a laminate, the oil resistance has been improved and it has been made possible to use it as a container for gasoline.
As described in the foregoing, the area of applications of the laminate of the polyolefin resin and a polyamide resin, a polyester resin or a saponified ethylene-vinyl acetate copolymer has been expanded. However, the polyolefin resin and the three types of other resins have poor affinity to each other, which brings about a drawback that the melt bonding can not readily be made. Various methods have been proposed to impart adhesive force.
As one of such methods, there has been a method in which a polyolefin resin is graft-modified with an unsaturated carboxylic acid or its anhydride, and the polyolefin resin and a polyamide resin are co-extruded to obtain a laminated sheet having excellent adhesion strength (Japanese Examined Patent Publication No. 43055/1976). In this case, a solution method or a slurry method may be employed as a method for the graft-modification of the polyolefin resin. However, a melt mixing method is preferred from an economical point of view.
However, when the polyolfin resin is graft-modified with an unsaturated carboxylic acid or its anhydride by means of a melt mixing method, gelation, fish eyes, color development, etc. which are fatal to films or sheets, are likely to occur. On the other hand, if the graft rate of the unsaturated carboxylic acid is maintained at a low level in order to avoid such problems, the adhesion strength of the resulting modified polyolefin will be inadequate. Under the circumstances, there have been some attempts for improvement of the adhesion by blending certain additives to the modified polyolefins. For instance, there may be mentioned a method of blending a rubber substance having a Mooney viscosity of from 40 to 150 to a modified polyolefin (Japanese Unexamined Patent Publication No. 98784/1976), a method of blending a low density polyethylene to a modified high density polyethylene (Japanese Unexamined Patent Publication No. 25845/1977) or a method of modifying a blend of a polyolefin and a rubber compound with an unsaturated carboxylic acid (Japanese Unexamined Patent Publication No. 8035/1977). These methods are effective for an improvement of the adhesion to some extent, but the effectiveness is not yet adequate. Besides, the blending with the rubber substance involves cumbersome mixing operation, which adds to the cost of the modified polyolefin.
Further, it has been proposed to use a composition obtained by incorporating a graft-copolymer of a polyolefin with an unsaturated carboxylic acid into an ethylene-vinyl acetate copolymer, for the preparation of a laminate with a polyamide layer (Japanese Examined Patent Publication No. 11032/1978). However, in this method, the polyolefin to be grafted is a conventional polyethylene or polypropylene having a high crystallinity, and therefore the adhesion with the polyamide layer is not necessarily adequate.
The present applicants have proposed in Japanese Unexamined Patent Publication No. 82/1979 to use a polyolefin composition comprising a crystalline polyolefin resin and an ethylene-.alpha.-olefin copolymer graft-modified with maleic anhydride, for the melt lamination of a polyolefin resin having a high crystallinity and a polyamide resin, a polyester resin or a saponified ethylene-vinyl acetate copolymer.
The present invention is an improvement over the earlier proposal.
In the melt lamination of an olefin resin and a polyamide resin, a polyester resin or a saponified ethylene-vinyl acetate copolymer, the olefin resin is required to have high interlayer adhesion strength and good transparency and not to lead to gelation, formation of fish eyes or undesirable color development, and it is further required to have good heat sealing properties when used in a laminated film and to be flexible when used at a low temperature.